Oxide charge induced high low junction emitter solar cell

ABSTRACT

A method of forming a high-low junction emitter silicon solar cell including the producing of an electron accumulation layer by oxide-charge-induction.

This application is a division of application Ser. No. 230,682, filedFeb. 2, 1981, now U.S. Pat. No. 4,343,962, which was a continuation ofSer. No. 057,648, filed on July 16, 1979, now abandoned.

THE INVENTION

The invention relates to a novel method of producing an improvedhigh-low junction silicon solar cell.

BACKGROUND OF THE INVENTION

As Brandhorst first noted, (H. W. Brandhorst, Jr., Record of 9thPhotovoltaic Specialists Conf. (IEEE, New York, 1972), p. 1.) the powerconversion efficiency η seen in silicon p-n junction solar cells isconsiderably less than the maximum theoretical value of η mainly becausethe open-circuit voltage V_(oc) is smaller than simple p-n junctiontheory predicts. Experiments on n⁺ -p silicon cells have shown that thisdiscrepancy in V_(oc) results from the dominance, in the nonilluminated(dark) cell, of the emitter recombination current J_(E) over the baserecombination current J_(B). In cells having base doping concentrationsof the order of 10¹⁷ cm⁻³, for which the largest values of V_(oc) areseen, J_(E) exceeds J_(B) by about an order of magnitude, rather thanbeing several orders of magnitude less than J_(B) as is predicted bysimple p-n junction theory. (F. A. Lindholm, A. Neugroschel, C. T. Sah,M. P. Goodlewski, and H. W. Brandhorst, Jr., IEEE Trans. ElectronDevices ED-24, 402 (1977). The excess J_(E) has been attributed to themechanisms of energy band-gap narrowing and lifetime degradation thataccompany heavy doping concentrations in the n⁺ emitter. (F. A. Lindholmand C. T. Sah, IEEE Trans. Electron. Devices ED-24, 299 (1977); C. T.Sah and F. A. Lindholm, IEEE Trans. Electron Devices ED-24, 358 (1977).

To supress J_(E) and thus raise the achievable value of V_(oc), a newstructure, the HLE junction solar cell, containing a high-how (H-L)junction in the emitter, has been proposed and its performance has beencalculated on theoretical grounds. (C. T. Sah, F. A. Lindholm, and J. G.Fossum, IEEE Trans. Electron Devices ED-25, 66 (1978); J. G. Fossum, F.A. Lindholm, and C. T. Sah, Tech. Digest 1977 Int. Electron Devices Mtg.(IEEE, New York, 1977), U.S. patent application Ser. No. 966,360, filedDec. 4, 1978, Lindholm, Fossum and Sah.)

OBJECTS OF THE INVENTION

It is an object of this invention to provide an improved method ofproducing an H-L junction in the emitter in a silicon solar cell.

A particular object of the invention is to provide a method by which anelectron accumulation layer is induced in the emitter of an n⁺ -psilicon solar cell to create an H-L junction by induction of an oxidecharge.

A general object of the invention is to provide in a silicon solar cellsubstantially complete suppression of the emitter recombination currentJ_(E) and to maximize V_(oc).

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features which are believed to be characteristic of thisinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation, together with further objects and advantages thereof, maybest be understood by reference to the following description taken inconnection with the accompanying drawing, in which:

FIG. 1 is a cross-sectional fragmentary schematic view of an OCI-HLEsolar cell made in accord with the novel method of this invention; and

FIGS. 2 and 3 are schematic views similar to FIG. 1 of prior art solarcells.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, FIG. 1 shows a n⁺ -n-p OCI-HLE solar cell 1which, according to the invention, comprises a p-type substrate 2, ann-type emitter comprising a low region 3 and a high electronaccumulation region 4, an oxide layer 5 on the illumination surface,positive oxide charge 6, shallow n⁺ contact diffusion 7, such diffusionunderlying each of the one or more aluminum or the like emitter contacts8, and an ohmic contact 9 for the p region. A p-n junction 10 existsbetween the emitter region 3 and the substrate region 2, and a high-howjunction 11 has been formed between the emitter regions 3 and 4.

The conventional n⁺ -p cell of FIG. 2 comprises p-type substrate region12 with an ohmic contact 13, a p-n junction 14, an n⁺ -type diffusedemitter 15 having an exposed, or antireflective layer coated,illuminatable surface 16 carrying aluminum or the like emitter contacts17.

FIG. 3 shows another prior art cell which comprises p-type substrate 18with an ohmic contact 19, n-type emitter 20, p-n junction 21, layer 22containing charges 23, and metal contacts 24.

While the emitter of the conventional cell is typically heavily doped(i.e. to about 10²⁰ cm⁻³), the emitter of the present invention ispreferably less heavily doped to less than about 10¹⁷ cm⁻³, andpreferably in the range of about 5×10¹⁵ to about 8×10¹⁶.

A heavily doped front layer, or emitter region, phosphorous or arsenicbeing common dopants has the disadvantage that the dark emitterrecombination is so large that it limits the open-circuit voltage toabout 600 mV maximum. This is well below the approximately 700 mVmaximum that would result if the dark emitter recombination current weresuppressed.

The structure of the invention--the oxide-charge-inducedhigh-low-junction emitter (OCI-HLE) solar cell--suppresses the darkemitter recombination current and enhances the short-circuit current,and thus enables the achievement of larger open-circuit voltages andpower conversion efficiencies than heretofore possible.

An illustration and discussion of the electron and hole concentrations,an energy band diagram, and other features and characteristics of HLEsolar cells, applicable to OCI-HLE solar cells formed in accord withthis invention, are included in the above mentioned U.S. applicationSer. No. 966,360, in connection with FIG. 3 thereof. In addition to theincrease of the open-circuit voltage V_(oc) there noted, the low holesurface recombination velocity Sp will result in only a negligiblerecombination of optically generated minority holes in the emitter atthe surface, which result in collection of almost all holes by a p-njunction 11.

An aluminum or the like metal contact 8 is in ohmic contact with theemitter and a shallow n⁺ diffusion 7 is made under this metalizedportion of the top surface area. The ohmic contact 8 should cover nomore than about 5 to 10% of the surface area. The n³⁰ diffusion 7 alsoprovides a high-low barrier with a small effective surface recombinationvelocity for holes. The induced accumulation layer 4 extends severalDebye lengths from the surface and can be as thin as 100 Å.

The high portion 4 of the high-low junction is thin and the lifetime inthe low-portion can be made long because of the low doping concentrationpresent in this portion. Consequently the transit time of holes acrossthe emitter region to the surface 5 is much less than the effectiverecombination lifetime in the bulk of the emitter. The lower portion 3of the high-low junction also serves to provide a large enough value oflateral sheet conductance that the lateral component of the seriesresistance can be made small enough to avoid degradation of the powerconversion efficiency. The HLE solar cell is useful both for one-sun andmultiple-sun applications.

A prior art cell disclosed in U.S. Pat. No. 4,144,094, which asdisclosed seems to have some similarities to the OCI-HLE cell of thepresent invention is shown in FIG. 3. The FIG. 3 cell, however, does notprovide an n⁺ contact diffusion underneath the metal contact 24. Sincethe recombination velocity at the silicon-metal interface is very largeand the doping in the low portion of the emitter 20 is low, the holerecombination current under the metalized portion of the surface 24 willbe very large. Also holes within distance about equal to the holediffusion from both sides of metal contact 24 will recombine under itwith large Sp. As a result the open-circuit voltage of the prior artcell of FIG. 3 will be limited to about 600 mV. Therefore a cell fromFIG. 3 would not represent an improvement of a conventional n⁺ -p cellwith diffused emitter from FIG. 2.

The problem of high Sp under the metal is solved in the present OCI-HLEstructure by a shallow n⁺ diffusion 7 which reduces high Sp under themetal to the much smaller value at the n⁺ -n interface.

An HLE cell in accord with the invention may be formed from commerciallyavailable eqitaxial silicon water material, or from less costly diffusedor ion implanted material, the silicon being crystalline, either mono-or polycrystalline, or from other semiconductor solar cell n-pmaterials. The OCI-HLE junction is then formed by introducing chargesinto the layer 5 covering the top surface. Most convenient realizationof this is to use a thermal silicon dioxide layer treated in dry oxygenat temperatures between 600° to 800° C. which treatment will result inthe presence of a positive charge at the surface. This silicon dioxidelayer will also serve as an antireflection coating. Surface layer 5 witha positive charge can be also realized using doped oxides, thermal plusevaporated or deposited oxides, thermal oxides plus nitrides, or usingother oxides, such as Cr and Ti oxides, and others.

Several runs of test cells have been made that demonstrate the desiredsuppression of the magnitude of the dark emitter current. In one suchrun, the starting material consisted of a 10 μm thick n-type layer ofresistivity ρ_(epi) ≃0.1 Ωcm grown epitaxially on a p-type substrate of300 μm thickness and resistivity ρ_(base) ≃0.1 Ωcm.

The substrate Z may have a resistivity of between about ρ=0.1 to 0.01Ωcm, being doped to between about 1×10¹⁹ and 5×10¹⁷ cm⁻³. Theresistivity of the emitter may range from about 0.1 to 1.0 Ωcm., thethickness of the emitter 3 may be between about 10 and 50 μm, and thedepth of the contact diffusion 7 may be between about 0.25 and 0.75 μm.

To fabricate OCI-HLE test cells according to the preferred embodiment,the wafer was oxidized for approximately 2 to 5 hours in dry O₂ with0.3% trichloroethylene, at from 1100° C. to 1150° C., to grow a 2500Åthick oxide layer, which was later etched to 1100Å to improve theantireflection properties. The temperature of 1100° C. to 1150° C. waschosen for this test cell to assure a good-quality oxide and a low valueof Sp. After oxidation the wafer was cooled from 1100° C. to 1000° C. atthe rate of 30° C. per hour and then cooled to 700° C. in three hours.

Openings through the oxide layer for the contact 8 are next provided byetching, the openings having an area equal to about 5 to 10% of thetotal oxide surface area. The wafer was then subjected to heating at900° C. for 20 minutes in the phosphorous diffusion furnace to providethe n⁺ contact diffusion or diffusions 7 in the layer 3 at each of theetched openings. The device was next heated in dry oxygen for from 2 to12 hours at 700° C. to increase the oxide charge density Q_(o).

The wafer is now provided with an emitter contact 8 at each opening inthe oxide layer 5. Aluminum is evaporated onto the wafer surface, and aphotolighographic pattern is applied permitting etching of the aluminumto leave the desired aluminum contacts 8, with their underlying contactdiffusions 7, and an appropriate network of interconnections for suchcontacts. Alternatively, a thin film of titanium may be evaporated onthe surface, followed by a film of silver, the total thickness of suchtitanium-silver coating being, for example, 1 μm. Such film is etched inthe same manner as described above.

Finally, the cell is annealed at about 400° C. for about 20 minutes in aforming gas, for example 10% H₂, 90% N₂.

According to the invention there is thus provided in a solar cell,suppression of dark emitter current, and increase of short circuitcurrent, without untenable series resistance, by the high-low junction11 in the emitter 3 together with low surface recombination velocity onthe nonmetallized portion of the illuminated emitter surface.

Further details of the invention are disclosed in the Applied PhysicsLetters, Vol. 33, pp 168-170, July 15, 1978, under the title "EmitterCurrent Suppression In A High-Low-Junction Emitter Solar Cell Using AnOxide-Charge-Induced Electron Accumulation Layer", by A. Neugroschel, F.A. Lindholm, S. C. Pao, and J. G. Fossum.

Further background and details of the invention are also set forth inthe publications referenced in the specification of the above identifiedapplication Ser. No. 966,360.

What is claimed and desired to be secured by U.S. Letters Patent is: 1.In a method of making a high-low junction emitter n-p solar cell whichcomprises providing a wafer including an n-type emitter and a p-typesubstrate, maintaining said wafer at an elevated temperature in anatmosphere consisting essentially of oxygen to form an oxide layer onsaid emitter, forming an opening in said layer, providing an n⁺diffusion under said opening, maintaining the wafer at a lower elevatedtemperature in dry oxygen to increase the oxide charge density, andapplying a conductive material in said opening in contact with said n⁺diffusion.
 2. The method of making a high-low junction emitter n-p solarcell comprising providing a silicon wafer having an n emitter and havinga p substrate provided with an ohmic surface contact, maintaining saidwafer in an oxidizing atmosphere at between about 1100° C. and 1150° C.for between about 2 and 12 hours, slowly cooling the wafer, providing anopening through the resultant oxide layer, maintaining the wafer atapproximately 900° C. in presence of phosphorous or n-type impuritiesfor approximately 20 minutes to produce an n⁺ diffusion under saidopening, maintaining said wafer at between about 600° and 800° C. in dryoxygen for about 2 hours, and applying a conductive contact in contactin with n⁺ diffusion.